The invention relates to a pressure limiting valve in a hydraulic machine. Pressure limiting valves of this type are known; they monitor and limit the maximum permissible pressure of the hydraulic fluid in a hydraulic machine. Pressure limiting valves of this type are predominantly used for limiting the filling pressure of hydraulic machines. However, the scavenging pressure of hydraulic machines is often monitored and limited by pressure limiting valves of this type. As soon as the pressure to be limited exceeds a specific maximum value, the pressure limiting valve opens and permits hydraulic fluid to flow out of the region under excess pressure into a region of the hydraulic machine in which a considerably lower pressure prevails.
In the basic form, pressure limiting valves of this type comprise a valve body having a spring under prestress. The spring presses the valve body onto a valve seat. In this case, there is equilibrium between the spring force and the forces from the pressures on either side of the valve seat. The pressure prevailing in the spring chamber assists the spring force. The pressure in the chamber on the other side of the valve, that is to say upstream of the valve disc, acts against the spring force and the pressure in the spring chamber.
Conflicting requirements are placed on pressure limiting valves of this type in hydraulic machines. For example, the valve seat should be as large as possible in order that the flow resistance of the opened pressure limiting valve is low. However, a large valve disc leads to high actuating forces, which have to be applied by a correspondingly large and powerful spring. Pressure limiting valves of such simple construction thus become very large.
Another problem in pressure limiting valves is that, in the case of pressure limiting valves with a conical valve disc, when the valve is opened, flow forces occur which act on the valve body with the effect of closing it. It has therefore already been proposed to avoid conical valve discs and, instead, to provide the valve bodies with flow guiding elements in the region of the valve seat, which are intended to have a beneficial effect on the flow.
Finally, in the case of pressure limiting valves, the spring force should be adjustable. In the case of a large and powerful valve spring, the result in this case is additional problems, which increase the outlay on construction and the space required by such valves.
The invention is based on the object of designing a pressure limiting valve in a hydraulic machine in such a way that, with a simple construction and reliable operation, it requires only little installation space and nevertheless has a very good flow characteristic.
This object is achieved with all of the features of Claim 1. It therefore comprises a pressure limiting valve in a hydraulic machine, having a valve seat which is fixed to the housing and forms the passage opening, having a valve body which comprises a valve disc matched to the valve seat and a tubular valve stem with an inner valve spring located therein and is guided in a bore in the housing of the hydraulic machine such that it can be displaced longitudinally and is sealed off, so that the bore in the housing, together with the valve body guided therein, forms a pressure-tight spring chamber for the inner valve spring, having an outer valve spring surrounding the valve stem on the outside, the two valve springs loading the valve body in the direction of the valve seat, having a valve chamber which extends transversely with respect to the longitudinal axis of the valve body and which, when the valve body is lifted off the valve seat, is connected via the flow opening to the chamber upstream of the valve seat, and having a hydraulic connection used for pressure equalization between the chamber upstream of the valve seat and the pressure-tight spring chamber.
In this case, all the features of the pressure limiting valve according to the invention act together in a mutually supplementary manner. Because the spring force is distributed to two valve springs, the valve springs are therefore connected in parallel, the installation space required for the pressure limiting valve in the hydraulic machine is reduced. The result here is the possibility that the outer valve spring surrounding the valve stem on the outside is dimensioned in accordance with the minimum pressure Setting of the pressure limiting valve, and therefore does not need to be adjustable. Only the inner valve spring, arranged in the tubular valve stem, then has to be provided with such a device for changing the prestress. This device can likewise be particularly compact, since it needs to be dimensioned only for the inner spring. A further reduction in the installation space results from the hydraulic connection, used for pressure equalization, between the chamber upstream of the valve seat and the pressure-tight spring chamber for the inner valve springs. A hydraulic connection of this type permits the pressure prevailing in the chamber upstream of the valve seat to become effective only on part of the end face of the valve body, while pressure equalization on the valve body is otherwise largely achieved. The actuating force for overcoming the closing force exerted by the inner valve spring is reduced considerably as a result. At the same time, the desired large flow cross section, which results from the diameter of the valve disc and the valve seat, is nevertheless available.
If, according to an advantageous development, the valve springs are formed as helical springs which are guided on the valve stem, the small amount of space required and the reliable operation of the pressure limiting valve according to the invention are promoted further as a result.
A particularly simple refinement is also achieved in that the valve disc forms a continuous base for the valve stem, which base acts as an active surface for the inner valve spring and has a pressure equalizing hole which forms the hydraulic connection between the chamber upstream of the valve seat and the pressure-tight spring chamber. Thus, the desired pressure equalization can readily be achieved by a simple hole.
The outer valve spring surrounding the valve stem on the outside can advantageously act on an annular collar, serving as an active surface, which is formed on the outside of the valve stem.
It is readily possible to guide the valve body of the pressure limiting valve according to the invention in the housing of the hydraulic machine itself. However, mounting is made considerably easier if, according to a further advantageous refinement, the pressure limiting valve according to the invention is provided with a tubular insert housing fixed in the housing of the hydraulic machine in a pressure-tight manner, in which the valve stem is guided in a pressure-tight manner and which, with an annular end facing the interior of the housing, acts as a supporting surface for the outer valve spring, the distance between the annular end of the insert housing and the annular collar formed on the outside of the valve stem determining the prestress of the outer valve spring.
In this case, the possibility results that the outer valve spring is dimensioned in accordance with the minimum pressure setting of the pressure limiting valve, that is to say does not have to be provided with its own device for setting the spring prestress.
According to another advantageous refinement, the inner valve spring can be supported on a continuous base of the insert housing, the distance between the continuous base of the insert housing and the continuous base of the valve stem determining the prestress of the inner valve spring. In this case, it is readily possible to provide a threaded hole in the continuous base of the insert housing, into which a setting screw can be screwed; the latter can be fixed by a locknut arranged on the outside and, as it is rotated, the prestress of the inner valve spring is changed. The result is, therefore, a simple possible adjustment, which likewise does not require any noticeable amount of space.
It is advantageous in this case if the end of the setting screw that faces the valve body projects into an intermediate sleeve with a hat-shaped cross section, which is used as an internal guide and supporting element for the inner valve spring. This intermediate sleeve makes it easier to rotate the setting screw.
The flow conditions as the pressure limiting valve according to the invention is opened are improved further by the fact that a flow guide element is formed on the valve disc, faces the chamber upstream of the valve seat and has transverse openings. The flow guide element can have a substantially cylindrical shape, the transverse openings being formed as holes or grooves.
By dimensioning the valve stem in relation to the valve seat, the characteristics of the pressure limiting valve according to the invention can be varied. It is preferable in the pressure limiting valve according to the invention if the external diameter d of the valve stem is smaller than the diameter D of the valve disc on the valve seat. The pressure limiting valve dimensioned in this way is suitable for installation conditions in which the pressure p1 of the hydraulic fluid in the chamber 2 upstream of the valve disc 7 is higher than the pressure p2 in the valve chamber. The valves installed in this way thus monitor the pressure p1 in the chamber upstream of the valve disc and open as soon as this pressure exceeds a specific maximum value.
Conversely, however, it is also possible to dimension the pressure limiting valve according to the invention in such a way that the external diameter of the valve stem is larger than the diameter of the valve disc on the valve seat. In this case, the pressure limiting valve according to the invention is installed in such a way that the pressure p2 in the valve chamber is the operationally higher pressure and is monitored with respect to reaching a maximum value.
Accordingly, the invention also relates to a hydraulic machine having a pressure limiting valve according to Claim 12, in which the external diameter d of the valve stem is smaller than the diameter D of the valve disc on the valve seat, the pressure limiting valve being installed in such a way that it monitors the pressure Pi in the chamber upstream of the valve seat with respect to reaching a maximum value.
However, the invention also relates to a hydraulic machine having a pressure limiting valve according to Claim 13, in which the external diameter of the valve stem is larger than the diameter of the valve disc on the valve seat, the pressure limiting valve being installed in such a way that it monitors the pressure p2 in the valve chamber with respect to reaching a maximum value.
The subject of FIG. 2 is the enlarged illustration of a detail from FIG. 1.